This invention relates to load current detecting apparatus of a direct current (DC) motor, more particularly apparatus for detecting effective load driving current (hereinafter merely called load current) which is equal to the difference between the armature current of the motor and the current necessary to accelerate or decelerate the motor.
In the operation of a hot strip mill or a tandem rolling mill of rolling shaped steel stocks, bar or wire shaped steel stocks, the shape and dimension of the products are often influenced by the variation in the interstand tension developed in the material being rolled. To eliminate such variation in the interstand tension, a looper control has been used. In the case of a hot strip mill, however, as the material being rolled is thick or as the shape is complicated, the looper control cannot be used. In such a case, so-called free tension control is used wherein the variation in the interstand tension is detected by detecting the variation of the torque of a mill driving DC motor.
FIG. 1 of the accompanying drawing shows one example of a free tension control device applied to a two stand tandem mill which comprises mill stands 1 and 2 respectively driven by DC motors 3 and 4, the speeds thereof being controlled by speed controllers 7 and 8.
The speed references of these two mill stands are given by speed ratio setters 11 and 12 which set the speed ratio of these stands, and a main speed setter 13 which sets the line speed. In the first mill stand, the speed controller 7 controls the speed of motor 3 such that the result of comparison of a speed reference NR.sub.1 and the output NF.sub.1 of the speed detector 5 which detects the speed of the motor made by a comparator 9 would be zero.
In the same manner, in the second stand, the speed controller 8 controls the speed of the motor 4 such that the result of comparison of a speed reference NR.sub.2 and the output NF.sub.2 of the speed detector 6 made by a comparator 10 would be zero.
The free tension control device 17 is constructed to calculate a speed correction quantity .DELTA.N.sub.1 by using the rolling force P.sub.1 of the first stand detected by a load cell 14, the armature current I.sub.1 of the motor 3 detected by a current detector 15, the terminal voltage V.sub.1 of the motor 3 detected by a voltage detector 16 and the speed NF.sub.1 of the motor 3 detected by the speed detector 5. The speed correction quantity .DELTA.N.sub.1 is applied to comparator 9 (which also acts as an adder or subtractor) to control the speed of the motor 3 for controlling the interstand tension between the first and second stands.
The free tension control device 17 operates according to the following principle.
In a rolling mill for rolling steel plate or the like the following equation holds between the rolling load P and the rolling torque G. EQU G/P=a (1)
The term a is generally termed a torque arm and is constant regardless of variation in the deformation resistance of the material.
In a case of a hot rolling mill, variation in the rolling force with reference to the variation in the tension applied to the material is generally much smaller than the variation in the rolling torque.
The free tension control is performed by utilizing the relationships described above. More particularly, the torque .DELTA.Gt.sub.1 caused by the tension between the first and second stands is expressed by the following equation (2): ##EQU1## where P.sub.01 represents the rolling force of the first stand under no tension condition during an interval in which the material 18 enters into the first stand and then reaches the second stand, G.sub.01 the rolling torque, P.sub.1 and P.sub.2 respectively represent the rolling force and the rolling torque after the material has entered into the second stand. In this case the rolling force represents a value detected by the load cell 14, and the rolling torque is calculated by the following equation: ##EQU2## where k.sub.1 : constant
R.sub.1 : armature resistance PA1 L.sub.1 : armature reactance. PA1 g.sub.1 : constant PA1 G.sub.t01 : target interstand tension between the first and second stands. PA1 g.sub.2 : constant. PA1 I.sub.R : load current PA1 I.sub.a : acceleration (or deceleration) current.
The speed of the first stand is corrected such that the tension torque .DELTA.G.sub.1 calculated by equation (2) will reach a certain target tension torque. The amount of speed correction .DELTA.N.sub.1 can be determined by the following equation: EQU .DELTA.N.sub.1 =g.sub.1 (G.sub.t01 -.DELTA.G.sub.t1) (4)
where
Under a steady rolling condition, should the tension varies so as to apply to the comparator 19 a first stand speed correction quantity .DELTA.N.sub.1 obtained by equation (1), the speed of the motor 3 would vary. At this time acceleration (or deceleration) current is flowing through the motor. .DELTA.G.sub.t1 obtained by substituting into equation (2) the torque G.sub.1 according to equation (3) under these conditions does not represent correct tension torque. In a tandem mill in which the line speed can be varied to any desired value by the main speed setter 13, the value of .DELTA.t.sub.1 obtained by using current I containing the acceleration or deceleration current does not represent the correct tension torque. To solve this problem, according to a prior art method, the torque .DELTA.G.sub.t1 has been determined by the following equation (5) by taking into consideration the acceleration or deceleration torque G.sub.al. ##EQU3## where GD.sup.2 : total GD.sup.2 of the rotating portion of the motor and the load thereof
However, since the acceleration of a DC motor lags, at the time of starting and completion of acceleration (or deceleration) phases of the acceleration (or deceleration) current and the acceleration (or deceleration) torque calculated by equation (6) are not equal so that at these points, even with equation (5) correct tension would not be obtained.
According to another method, sampling control is effected for an acceleration (or deceleration) caused by the speed correction quantity .DELTA.N.sub.1 and the next .DELTA.G.sub.t1 is not calculated untill speed correction is completed. This results in a coarser sampling pitch so that it is impossible to sufficiently increase the response speed of the free tension control.
As above described the most serious problem of the free tension control lies in the control of acceleration and deceleration, and the present invention contemplates solution of this problem.
As above described, the armature current I equals to the sum of the acceleration (or deceleration) current and the load current. Thus: EQU I=I.sub.R +I.sub.a ( 7)
where
The factor which is necessary to calculate the rolling torque is the current I.sub.R corresponding to the load torque. Accordingly when I.sub.1 (FIG. 1) is substituted by IR.sub.1 a correct free tension control can be realized. Thus, according to this invention, there is provided apparatus for detecting load current I.sub.R obtained by subtracting the acceleration (or deceleration) current Ia from the armature current.
As shown in equation (6), the factors that determine the acceleration or deceleration torque are the moment of inertia GD.sup.2 and the rate of acceleration (or deceleration) dN/dt.